Multi-layer fabrication techniques are so far the most promising and versatile methods for fabricating truly three-dimensional microstructures with high-aspect-ratios for microelectromechanical systems (MEMS). U.S. Pat. No. 5,190,637, issued on Mar. 2, 1993 to Henry Guckel, and U.S. Pat. No. 6,027,630, issued on Feb. 22, 2000 to Adam L. Cohen, teach two fabrication methods for forming three-dimensional metal microstructures by using multi-layer electrochemical deposition. The only difference between the two methods is the way to perform the selective electrochemical deposition of metals. The essential elements of these two methods are described as follows.
(1) Three-dimensional microstructures are fabricated by using a successive layer-by-layer approach, meaning that a given layer can only be built after the pervious layer is completed.
(2) Each layer contains at least two different materials. In the case of a two-material system, one is a sacrificial material and the other is a structural material.
(3) Both structural material and sacrificial material are fabricated by using electrochemical deposition methods such as electrodeposition.
(4) After the deposition of both materials for building a given layer, a planarization process is used to machine the deposited materials to a predetermined thickness and to a flat and smooth surface as a base for building a next layer on it.
(5) After all layers have been built, the sacrificial material is etched to form a three-dimensional microstructure.
Although the above two multilayer electrochemical fabrication methods are practical for building truly three-dimensional microstructures, there are at least five (5) major drawbacks inherently associated with them, which are:
(1) Low throughput. Production throughput becomes an issue as the number of layers in the microdevices increases. This is because the multilayer approach is a successive process. A given layer can only be built after the pervious layer is completed. The two main risks associated with a successive process are: 1) When one layer meets fabrication problems, the fabrication process flow for this build has to be suspended until the problems are solved; and 2) If a build has to be discarded due to various reasons during fabrication, all previously fabricated layers have to be wasted.
(2) Low yield. In reality, each layer inevitably contains some defects. These defects may distribute at different locations on each layer. Therefore, the final yield of acceptable structures is an issue.
(3) Limited material selection. Electrochemical deposition can only be used to fabricate metals and alloys. It can not or can not easily make many important engineering and semiconductor materials such as stainless steel, aluminum and its alloys, titanium and its alloys, silicon, and non-metal materials. Further, although electrodeposition can be used to make many metals and alloys, only a small portion of them can be made practically and economically for mass production. Even among the metals and alloys that can be readily electrodeposited, there are those whose material properties are not as desired as those of the corresponding bulk materials. Therefore the electrochemical fabrication methods taught in the two patents have limited applications.
(4) Layer thickness limitation. The maximum metal layer thickness is restricted by the mask thickness (e.g., photoresist). The ability to select the layer thickness freely is crucial to the multilayer approach. For example, in some cases, several successive thin layers may have to be deposited to reach a required thickness if a single thick layer cannot be formed. Obviously, this would increase production time, decrease production throughput and lower product yield.
(5) Geometry limitation. The multi-layer electrochemical fabrication methods can manufacture complex three-dimensional metal microstructures. However, there exist some types of shapes which can not be fabricated. This geometry limitation is related to the etching of sacrificial material. If sacrificial material can not be removed (e.g., in a sealed cavity with no etching access paths) or can not be removed easily (e.g., in a long, narrow channel), microstructures with these shapes can not be made with the two methods.
U.S. Pat. No. 6,332,568 to Todd R. Christenson issued on Dec. 25, 2001 teaches a method to make three-dimensional microstructures via diffusion bonding of subassemblies to form a united structure. This method allows the parallel manufacture of a series of subassemblies which compose a microstructure or micromachine. Each subassembly is built on a separate substrate. Then according to a predetermined configuration of the microstructure, the second subassembly is diffusion bonded onto the first subassembly. The substrate of the second subassembly is separated from the bonded two subassemblies after diffusion bonding. Therefore the substrate of a subassembly can be viewed as a sacrificial material as the substrate needs to be removed after one bonding operation is done. The above two steps (diffusion bonding and separation) are repeated until the all subassemblies are bonded to form a united structure. Compared with the multi-layer electrochemical fabrication approaches, the advantage of Christenson's approach is that all the required subassemblies for a microstructure can be built separately. However, the bonding of the subassemblies is still a successive process, meaning that the previous subassembly must be bonded before a given subassembly can be bonded. Diffusion bonding involves heat and pressure. Assume that a microstructure comprises ten subassemblies. Only the last (10th) subassembly undergoes heat and pressure treatment one time. All other nine subassemblies undergo heat and pressure treatment at least two times. The first two (1st and 2nd) subassemblies even undergo heat and pressure treatment nine times. The heat and pressure treatment may change the material properties of a microstructure or may distort the components of a microstructure. Therefore, practically this bonding method can only work for the very limited number of subassemblies although in theory there is no limit on the number of subassemblies that can be joined.
In addition, as each of two subassemblies to be bonded contains a substrate which increases the difficulty for the alignment of micro-components on the two substrates before bonding. The reason that Christenson's approach has to use a substrate as part of a subassembly is that the substrate works as a mechanical support for micro-components on it. Without this substrate, all components on the substrate will fall apart. It is very difficult to align two subassembly with their substrates as micro-components are located between two substrates during alignment. The micro-components on the two substrates could be damaged due to this difficult alignment as the components are not protected and can not be protected. After two subassemblies are bonded together, an extra process step has to be operated to separate one of the substrates, which not only increases process time and cost, but also may damage the bonded micro-components.